Interpretive Summary: The genes in the SABATH family of methyltransferases have important biological functions in plants such as regulating hormones, signaling molecules and other metabolites. While all previously characterized SABATH genes were isolated from flowering plants, in this study, we report on the isolation and functional characterization of SABATH genes from white spruce (Picea glauca [Moench] Voss), a conifer. These genes were cloned and expressed in E. coli. One of the SABATH genes displayed the highest level of activity with the plant hormone indole-3-acetic acid (IAA). We therefore named this gene, PgIAMT1. Structural modeling of PgIAMT1 revealed that the active site of PgIAMT1 is highly similar to other characterized IAMTs from flowering plants. PgIAMT1 was expressed in multiple tissues with the highest level of expression detected in embryonic tissues at the early stage. The PgIAMT1 expression level dropped as the embryo matured.

Technical Abstract:
Known members of the plant SABATH family of methyltransferases have
important biological functions by methylating hormones, signaling molecules and other metabolites. While all previously characterized SABATH genes were isolated from angiosperms, in this article, we report on the isolation and functional characterization of SABATH genes from white spruce (Picea glauca [Moench] Voss), a gymnosperm. Through EST database search, three genes that encode proteins significantly homologous to known SABATH proteins were identified from white spruce. They were named
PgSABATH1, PgSABATH2 and PgSABATH3, respectively. Full length cDNAs of these three genes were cloned and expressed in E. coli. The E. coli-expressed recombinant proteins were tested for methyltransferase activity with a large number of compounds.
While no activity was detected for PgSABATH2 and PgSABATH3, PgSABATH1 displayed the highest level of catalytic activity with indole-3-acetic acid (IAA). PgSABATH1 was therefore renamed PgIAMT1. Under steady-state conditions, PgIAMT1 exhibited apparent Km values of 18.2 µM for IAA. Homology-based structural
modeling of PgIAMT1 revealed that the active site of PgIAMT1 is highly similar to other characterized IAMTs from angiosperms. PgIAMT1 showed expression in multiple tissues with the highest level of expression detected in embryonic tissues. During somatic embryo maturation, a significant reduction in PgIAMT1 transcript levels was observed when developing cotyledons become apparent which is indicative of mature embryos. The biological roles of white spruce SABATH genes, especially those of PgIAMT1, and
the evolution of the SABATH family are discussed.